Conventionally, fiber-reinforced composites for use in the field of aeronautics and space comprise carbon or ceramics such as silicon carbide formed as a matrix which is reinforced with carbon fiber or ceramic fiber such as silicon carbide fiber. Of such fiber-reinforced composites, a carbon-fiber reinforced composite exhibits a superior specific mechanical strength (strength per unit weight) and higher fracture toughness even at high temperatures above 1500.degree. C.
Since the carbon-fiber reinforced composite includes carbon fiber, however, the composite has a disadvantage that is has poor oxidation-resistance and poor erosion-resistance at high temperatures.
Thus, in order to eliminate such a disadvantage, a coated layer made of silicon carbide or silicon nitride having excellent oxidation-resistance is formed on a surface. However, since the thermal expansion coefficient of the carbon-fiber reinforced composite is extremely low as compared to that of the coated layer, there is a problem that thermal-cracks are produced in the coated layer during a heat cycle. Consequently, it has been difficult to provide conventional fiber-reinforced composites having good oxidation-resistance and good erosion-resistance.
Further, regarding other reinforcing materials, there are ceramic fiber reinforced composites in which the matrix of silicon carbide or silicon nitride is reinforced with ceramic fiber such as silicon carbide fiber. Since both the reinforcing fiber and the matrix of the ceramic-fiber reinforced composites are formed of substances having good oxidation-resistance and good erosion-resistance, the composites are excellent in oxidation-resistance and good in erosion-resistance at high temperatures and require no oxidation-resistive coated layer. Taking silicon carbide fiber as an example, however, because there is a phenomenon that the mechanical strength of silicon carbide fiber decreases at approximately 1200.degree. C. or more, a composite using these fibers has a poor mechanical strength characteristic at high temperatures.